Semiconductor device comprising a superlattice channel vertically stepped above source and drain regions

ABSTRACT

A semiconductor device may include a semiconductor substrate and at least one metal oxide semiconductor field-effect transistor (MOSFET). The at least one MOSFET may include spaced apart source and drain regions in the semiconductor substrate, and a superlattice channel including a plurality of stacked groups of layers on the semiconductor substrate between the source and drain regions. The superlattice channel may have upper surface portions vertically stepped above adjacent upper surface portions of the source and drain regions. Each group of layers of the superlattice channel may include a plurality of stacked base semiconductor monolayers defining a base semiconductor portion and an energy band-modifying layer thereon. The energy-band modifying layer may include at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor. The at least one MOSFET may additionally include a gate overlying the superlattice channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/647,069 filed Aug. 22, 2003, which in turn is acontinuation-in-part of U.S. patent application Ser. Nos. 10/603,696 and10/603,621, both filed on Jun. 26, 2003, the entire disclosures of whichare hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductors, and, moreparticularly, to semiconductors having enhanced properties based uponenergy band engineering and associated methods.

BACKGROUND OF THE INVENTION

Structures and techniques have been proposed to enhance the performanceof semiconductor devices, such as by enhancing the mobility of thecharge carriers. For example, U.S. Patent Application No. 2003/0057416to Currie et al. discloses strained material layers of silicon,silicon-germanium, and relaxed silicon and also including impurity-freezones that would otherwise cause performance degradation. The resultingbiaxial strain in the upper silicon layer alters the carrier mobilitiesenabling higher speed and/or lower power devices. Published U.S. PatentApplication No. 2003/0034529 to Fitzgerald et al. discloses a CMOSinverter also based upon similar strained silicon technology.

U.S. Pat. No. 6,472,685 B2 to Takagi discloses a semiconductor deviceincluding a silicon and carbon layer sandwiched between silicon layersso that the conduction band and valence band of the second silicon layerreceive a tensile strain. Electrons having a smaller effective mass, andwhich have been induced by an electric field applied to the gateelectrode, are confined in the second silicon layer, thus, an n-channelMOSFET is asserted to have a higher mobility.

U.S. Pat. No. 4,937,204 to Ishibashi et al. discloses a superlattice inwhich a plurality of layers, less than eight monolayers, and containinga fraction or a binary compound semiconductor layers, are alternatelyand epitaxially grown. The direction of main current flow isperpendicular to the layers of the superlattice.

U.S. Pat. No. 5,357,119 to Wang et al. discloses a Si—Ge short periodsuperlattice with higher mobility achieved by reducing alloy scatteringin the superlattice. Along these lines, U.S. Pat. No. 5,683,934 toCandelaria discloses an enhanced mobility MOSFET including a channellayer comprising an alloy of silicon and a second materialsubstitutionally present in the silicon lattice at a percentage thatplaces the channel layer under tensile stress.

U.S. Pat. No. 5,216,262 to Tsu discloses a quantum well structurecomprising two barrier regions and a thin epitaxially grownsemiconductor layer sandwiched between the barriers. Each barrier regionconsists of alternate layers of SiO₂/Si with a thickness generally in arange of two to six monolayers. A much thicker section of silicon issandwiched between the barriers.

An article entitled “Phenomena in silicon nanostructure devices” also toTsu and published online Sep. 6, 2000 by Applied Physics and MaterialsScience & Processing, pp. 391-402 discloses a semiconductor-atomicsuperlattice (SAS) of silicon and oxygen. The Si/O superlattice isdisclosed as useful in a silicon quantum and light-emitting devices. Inparticular, a green electroluminescence diode structure was constructedand tested. Current flow in the diode structure is vertical, that is,perpendicular to the layers of the SAS. The disclosed SAS may includesemiconductor layers separated by adsorbed species such as oxygen atoms,and CO molecules. The silicon growth beyond the adsorbed monolayer ofoxygen is described as epitaxial with a fairly low defect density. OneSAS structure included a 1.1 nm thick silicon portion that is abouteight atomic layers of silicon, and another structure had twice thisthickness of silicon. An article to Luo et al. entitled “Chemical Designof Direct-Gap Light-Emitting Silicon” published in Physical ReviewLetters, Vol. 89, No. 7 (Aug. 12, 2002) further discusses the lightemitting SAS structures of Tsu.

Published International Application WO 02/103,767 A1 to Wang, Tsu andLofgren, discloses a barrier building block of thin silicon and oxygen,carbon, nitrogen, phosphorous, antimony, arsenic or hydrogen to therebyreduce current flowing vertically through the lattice more than fourorders of magnitude. The insulating layer/barrier layer allows for lowdefect epitaxial silicon to be deposited next to the insulating layer.

Published Great Britain Patent Application 2,347,520 to Mears et al.discloses that principles of Aperiodic Photonic Band-Gap (APBG)structures may be adapted for electronic bandgap engineering. Inparticular, the application discloses that material parameters, forexample, the location of band minima, effective mass, etc, can betailored to yield new aperiodic materials with desirable band-structurecharacteristics. Other parameters, such as electrical conductivity,thermal conductivity and dielectric permittivity or magneticpermeability are disclosed as also possible to be designed into thematerial.

Despite considerable efforts at materials engineering to increase themobility of charge carriers in semiconductor devices, there is still aneed for greater improvements. Greater mobility may increase devicespeed and/or reduce device power consumption. With greater mobility,device performance can also be maintained despite the continued shift tosmaller device features.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a semiconductor device including one ormore MOSFETS having relatively high charge carrier mobility and relatedmethods.

This and other objects, features, and advantages in accordance with thepresent invention are provided by a semiconductor device which mayinclude a semiconductor substrate, and at least one metal oxidesemiconductor field-effect transistor (MOSFET). More particularly, theat least one MOSFET may include spaced apart source and drain regions inthe semiconductor substrate, and a superlattice channel including aplurality of stacked groups of layers on the semiconductor substratebetween the source and drain regions. Furthermore, the superlatticechannel may have upper surface portions vertically stepped aboveadjacent upper surface portions of the source and drain regions. Moreparticularly, each group of layers of the superlattice channel mayinclude a plurality of stacked base semiconductor monolayers defining abase semiconductor portion and an energy band-modifying layer thereon.Also, the energy-band modifying layer may include at least onenon-semiconductor monolayer constrained within a crystal lattice ofadjacent base semiconductor. The at least one MOSFET may additionallyinclude a gate overlying the superlattice channel.

The semiconductor device may also include an underlying portion of thesubstrate aligned with the superlattice channel and also beingvertically stepped above the source and drain regions. Furthermore, thegate may include a gate oxide layer and a gate electrode thereover, andthe superlattice channel may be aligned with the gate electrode. Thegate may also include a sidewall spacers on opposing sides of the gateelectrode, and in accordance with an alternate embodiment thesuperlattice channel may be aligned with the sidewall spacers.

In addition, the at least one MOSFET may be a plurality thereof, and thesemiconductor device may further include an isolation region in thesemiconductor substrate between adjacent MOSFETs. A contact layer mayalso be included on at least one of the source and drain regions.

More specifically, the superlattice channel may have a common energyband structure therein, and it may also have a higher charge carriermobility than would otherwise be present. Each base semiconductorportion may comprise silicon, and each energy band-modifying layer maycomprise oxygen. Further, each energy band-modifying layer may be asingle monolayer thick, and each base semiconductor portion may be lessthan eight monolayers thick.

The superlattice may further have a substantially direct energy bandgap,and it may also include a base semiconductor cap layer on an uppermostgroup of layers. In one embodiment, all of the base semiconductorportions may be a same number of monolayers thick. In accordance with analternate embodiment, at least some of the base semiconductor portionsmay be a different number of monolayers thick. In addition, each energyband-modifying layer may include a non-semiconductor selected from thegroup consisting of oxygen, nitrogen, fluorine, and carbon-oxygen, forexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic cross-sectional view of a semiconductor device inaccordance with the present invention.

FIG. 2 is a greatly enlarged schematic cross-sectional view of thesuperlattice as shown in FIG. 1.

FIG. 3 is a perspective schematic atomic diagram of a portion of thesuperlattice shown in FIG. 1.

FIG. 4 is a greatly enlarged schematic cross-sectional view of anotherembodiment of a superlattice that may be used in the device of FIG. 1.

FIG. 5A is a graph of the calculated band structure from the gamma point(G) for both bulk silicon as in the prior art, and for the 4/1 Si/Osuperlattice as shown in FIGS. 1-3.

FIG. 5B is a graph of the calculated band structure from the Z point forboth bulk silicon as in the prior art, and for the 4/1 Si/O superlatticeas shown in FIGS. 1-3.

FIG. 5C is a graph of the calculated band structure from both the gammaand Z points for both bulk silicon as in the prior art, and for the5/1/3/1 Si/O superlattice as shown in FIG. 4.

FIGS. 6A-6E are a series of schematic cross-sectional diagramsillustrating a method for making the semiconductor device of FIG. 1.

FIGS. 7A-7D are a series of schematic cross-sectional diagramsillustrating a method for making an alternate embodiment of thesemiconductor device of FIG. 1.

FIG. 8 is a schematic cross-sectional diagram illustrating a completedsemiconductor device formed using the method steps illustrated in FIGS.7A-7E.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout, and prime and multiple primenotation are used to indicate similar elements in alternate embodiments.

The present invention relates to controlling the properties ofsemiconductor materials at the atomic or molecular level to achieveimproved performance within semiconductor devices. Further, theinvention relates to the identification, creation, and use of improvedmaterials for use in the conduction paths of semiconductor devices.

Applicants theorize, without wishing to be bound thereto, that certainsuperlattices as described herein reduce the effective mass of chargecarriers and that this thereby leads to higher charge carrier mobility.Effective mass is described with various definitions in the literature.As a measure of the improvement in effective mass Applicants use a“conductivity reciprocal effective mass tensor”, M_(e) ⁻¹ and M_(h) ⁻¹for electrons and holes respectively, defined as:${M_{e,{i\quad j}}^{- 1}( {E_{F},T} )} = \frac{\sum\limits_{E > E_{F}}{\int_{B.Z.}{( {\nabla_{k}{E( {k,n} )}} )_{i}( {\nabla_{k}{E( {k,n} )}} )_{j}\frac{\partial{f( {{E( {k,n} )},E_{F},T} )}}{\partial E}\quad{\mathbb{d}^{3}k}}}}{\sum\limits_{E > E_{F}}{\int_{B.Z.}{{f( {{E( {k,n} )},E_{F},T} )}\quad{\mathbb{d}^{3}k}}}}$for electrons and:${M_{h,{i\quad j}}^{- 1}( {E_{F},T} )} = \frac{\underset{E < E_{F}}{- \sum}{\int_{B.Z.}{( {\nabla_{k}{E( {k,n} )}} )_{i}( {\nabla_{k}{E( {k,n} )}} )_{j}\frac{\partial{f( {{E( {k,n} )},E_{F},T} )}}{\partial E}\quad{\mathbb{d}^{3}k}}}}{\sum\limits_{E < E_{F}}{\int_{B.Z.}{( {1 - {f( {{E( {k,n} )},E_{F},T} )}} )\quad{\mathbb{d}^{3}k}}}}$for holes, where f is the Fermi-Dirac distribution, E_(F) is the Fermienergy, T is the temperature, E(k,n) is the energy of an electron in thestate corresponding to wave vector k and the n^(th) energy band, theindices i and j refer to Cartesian coordinates x, y and z, the integralsare taken over the Brillouin zone (B.Z.), and the summations are takenover bands with energies above and below the Fermi energy for electronsand holes respectively.

Applicants' definition of the conductivity reciprocal effective masstensor is such that a tensorial component of the conductivity of thematerial is greater for greater values of the corresponding component ofthe conductivity reciprocal effective mass tensor. Again Applicantstheorize without wishing to be bound thereto that the superlatticesdescribed herein set the values of the conductivity reciprocal effectivemass tensor so as to enhance the conductive properties of the material,such as typically for a preferred direction of charge carrier transport.The inverse of the appropriate tensor element is referred to as theconductivity effective mass. In other words, to characterizesemiconductor material structures, the conductivity effective mass forelectrons/holes as described above and calculated in the direction ofintended carrier transport is used to distinguish improved materials.

Using the above-described measures, one can select materials havingimproved band structures for specific purposes. One such example wouldbe a superlattice 25 material for a channel region in a semiconductordevice. A planar MOSFET 20 including the superlattice 25 in accordancewith the invention is now first described with reference to FIG. 1. Oneskilled in the art, however, will appreciate that the materialsidentified herein could be used in many different types of semiconductordevices, such as discrete devices and/or integrated circuits.

The illustrated MOSFET 20 includes a substrate 21, lightly dopedsource/drain extension regions 22, 23, more heavily doped source/drainregions 26, 27, and a channel region therebetween provided by thesuperlattice 25. Source/drain silicide layers 30, 31 and source/draincontacts 32, 33 overlie the source/drain regions, as will be appreciatedby those skilled in the art. A gate 35 illustratively includes a gateinsulating layer 37 adjacent the channel provided by the superlattice25, and a gate electrode layer 36 on the gate insulating layer. Sidewallspacers 40, 41 are also provided in the illustrated MOSFET 20, as wellas a silicide layer 34 on the gate electrode layer 36.

In accordance with the present invention, the superlattice 25 materialforming the channel has an upper surface portion vertically steppedabove adjacent upper surface portions of the source/drain extensionregions 22, 23 and the source/drain regions 26, 27, as shown.Furthermore, in the illustrated embodiment an underlying portion 24 ofthe substrate is aligned with the superlattice 25. That is, the lateralboundaries of the superlattice 25 and underlying portion 24 arecoterminous as shown. The superlattice 25 material is similarly alignedwith the gate electrode 36.

Applicants have identified improved materials or structures for thechannel region of the MOSFET 20. More specifically, the Applicants haveidentified materials or structures having energy band structures forwhich the appropriate conductivity effective masses for electrons and/orholes are substantially less than the corresponding values for silicon.

Referring now additionally to FIGS. 2 and 3, the materials or structuresare in the form of a superlattice 25 whose structure is controlled atthe atomic or molecular level and may be formed using known techniquesof atomic or molecular layer deposition. The superlattice 25 includes aplurality of layer groups 45 a-45 n arranged in stacked relation, asperhaps best understood with specific reference to the schematiccross-sectional view of FIG. 2.

Each group of layers 45 a-45 n of the superlattice 25 illustrativelyincludes a plurality of stacked base semiconductor monolayers 46defining a respective base semiconductor portion 46 a-46 n and an energyband-modifying layer 50 thereon. The energy band-modifying layers 50 areindicated by stippling in FIG. 2 for clarity of illustration.

The energy-band modifying layer 50 illustratively includes onenon-semiconductor monolayer constrained within a crystal lattice ofadjacent base semiconductor portions. In other embodiments, more thanone such monolayer may be possible. It should be noted that referenceherein to a non-semiconductor or semiconductor monolayer means that thematerial used for the monolayer would be a non-semiconductor orsemiconductor if formed in bulk. That is, a single monolayer of amaterial, such as semiconductor, may not necessarily exhibit the sameproperties that it would if formed in bulk or in a relatively thicklayer, as will be appreciated by those skilled in the art.

Applicants theorize without wishing to be bound thereto that energyband-modifying layers 50 and adjacent base semiconductor portions 46a-46 n cause the superlattice 25 to have a lower appropriateconductivity effective mass for the charge carriers in the parallellayer direction than would otherwise be present. Considered another way,this parallel direction is orthogonal to the stacking direction. Theband modifying layers 50 may also cause the superlattice 25 to have acommon energy band structure.

It is also theorized that the semiconductor device, such as theillustrated MOSFET 20, enjoys a higher charge carrier mobility basedupon the lower conductivity effective mass than would otherwise bepresent. In some embodiments, and as a result of the band engineeringachieved by the present invention, the superlattice 25 may further havea substantially direct energy bandgap that may be particularlyadvantageous for opto-electronic devices, for example, as described infurther detail below.

As will be appreciated by those skilled in the art, the source/drainregions 22, 23 and gate 35 of the MOSFET 20 may be considered as regionsfor causing the transport of charge carriers through the superlattice ina parallel direction relative to the layers of the stacked groups 45a-45 n. Other such regions are also contemplated by the presentinvention.

The superlattice 25 also illustratively includes a cap layer 52 on anupper layer group 45 n. The cap layer 52 may comprise a plurality ofbase semiconductor monolayers 46. The cap layer 52 may have between 2 to100 monolayers of the base semiconductor, and, more preferably between10 to 50 monolayers.

Each base semiconductor portion 46 a-46 n may comprise a basesemiconductor selected from the group consisting of Group IVsemiconductors, Group III-V semiconductors, and Group II-VIsemiconductors. Of course, the term Group IV semiconductors alsoincludes Group IV-IV semiconductors, as will be appreciated by thoseskilled in the art. More particularly, the base semiconductor maycomprise at least one of silicon and germanium, for example.

Each energy band-modifying layer 50 may comprise a non-semiconductorselected from the group consisting of oxygen, nitrogen, fluorine, andcarbon-oxygen, for example. The non-semiconductor is also desirablythermally stable through deposition of a next layer to therebyfacilitate manufacturing. In other embodiments, the non-semiconductormay be another inorganic or organic element or compound that iscompatible with the given semiconductor processing as will beappreciated by those skilled in the art. More particularly, the basesemiconductor may comprise at least one of silicon and germanium, forexample.

It should be noted that the term monolayer is meant to include a singleatomic layer and also a single molecular layer. It is also noted thatthe energy band-modifying layer 50 provided by a single monolayer isalso meant to include a monolayer wherein not all of the possible sitesare occupied. For example, with particular reference to the atomicdiagram of FIG. 3, a 4/1 repeating structure is illustrated for siliconas the base semiconductor material, and oxygen as the energyband-modifying material. Only half of the possible sites for oxygen areoccupied.

In other embodiments and/or with different materials this one halfoccupation would not necessarily be the case as will be appreciated bythose skilled in the art. Indeed it can be seen even in this schematicdiagram, that individual atoms of oxygen in a given monolayer are notprecisely aligned along a flat plane as will also be appreciated bythose of skill in the art of atomic deposition. By way of example, apreferred occupation range is from about one-eighth to one-half of thepossible oxygen sites being full, although other numbers may be used incertain embodiments.

Silicon and oxygen are currently widely used in conventionalsemiconductor processing, and, hence, manufacturers will be readily ableto use these materials as described herein. Atomic or monolayerdeposition is also now widely used. Accordingly, semiconductor devicesincorporating the superlattice 25 in accordance with the invention maybe readily adopted and implemented, as will be appreciated by thoseskilled in the art.

It is theorized without Applicants wishing to be bound thereto, that fora superlattice, such as the Si/O superlattice, for example, that thenumber of silicon monolayers should desirably be seven or less so thatthe energy band of the superlattice is common or relatively uniformthroughout to achieve the desired advantages. The 4/1 repeatingstructure shown in FIGS. 2 and 3, for Si/O has been modeled to indicatean enhanced mobility for electrons and holes in the X direction. Forexample, the calculated conductivity effective mass for electrons(isotropic for bulk silicon) is 0.26 and for the 4/1 SiO superlattice inthe X direction it is 0.12 resulting in a ratio of 0.46. Similarly, thecalculation for holes yields values of 0.36 for bulk silicon and 0.16for the 4/1 Si/O superlattice resulting in a ratio of 0.44.

While such a directionally preferential feature may be desired incertain semiconductor devices, other devices may benefit from a moreuniform increase in mobility in any direction parallel to the groups oflayers. It may also be beneficial to have an increased mobility for bothelectrons or holes, or just one of these types of charge carriers aswill be appreciated by those skilled in the art.

The lower conductivity effective mass for the 4/1 Si/O embodiment of thesuperlattice 25 may be less than two-thirds the conductivity effectivemass than would otherwise occur, and this applies for both electrons andholes. Of course, the superlattice 25 may further comprise at least onetype of conductivity dopant therein as will also be appreciated by thoseskilled in the art.

Indeed, referring now additionally to FIG. 4, another embodiment of asuperlattice 25′ in accordance with the invention having differentproperties is now described. In this embodiment, a repeating pattern of3/1/5/1 is illustrated. More particularly, the lowest base semiconductorportion 46 a′ has three monolayers, and the second lowest basesemiconductor portion 46 b′ has five monolayers. This pattern repeatsthroughout the superlattice 25′ The energy band-modifying layers 50′ mayeach include a single monolayer. For such a superlattice 25′ includingSi/O, the enhancement of charge carrier mobility is independent oforientation in the plane of the layers. Those other elements of FIG. 4not specifically mentioned are similar to those discussed above withreference to FIG. 2 and need no further discussion herein.

In some device embodiments, all of the base semiconductor portions of asuperlattice may be a same number of monolayers thick. In otherembodiments, at least some of the base semiconductor portions may be adifferent number of monolayers thick. In still other embodiments, all ofthe base semiconductor portions may be a different number of monolayersthick.

In FIGS. 5A-5C band structures calculated using Density FunctionalTheory (DFT) are presented. It is well known in the art that DFTunderestimates the absolute value of the bandgap. Hence all bands abovethe gap may be shifted by an appropriate “scissors correction.” Howeverthe shape of the band is known to be much more reliable. The verticalenergy axes should be interpreted in this light.

FIG. 5A shows the calculated band structure from the gamma point (G) forboth bulk silicon (represented by continuous lines) and for the 4/1 Si/Osuperlattice 25 as shown in FIGS. 1-3 (represented by dotted lines). Thedirections refer to the unit cell of the 4/1 Si/O structure and not tothe conventional unit cell of Si, although the (001) direction in thefigure does correspond to the (001) direction of the conventional unitcell of Si, and, hence, shows the expected location of the Si conductionband minimum. The (100) and (010) directions in the figure correspond tothe (110) and (−110) directions of the conventional Si unit cell. Thoseskilled in the art will appreciate that the bands of Si on the figureare folded to represent them on the appropriate reciprocal latticedirections for the 4/1 Si/O structure.

It can be seen that the conduction band minimum for the 4/1 Si/Ostructure is located at the gamma point in contrast to bulk silicon(Si), whereas the valence band minimum occurs at the edge of theBrillouin zone in the (001) direction which we refer to as the Z point.One may also note the greater curvature of the conduction band minimumfor the 4/1 Si/O structure compared to the curvature of the conductionband minimum for Si owing to the band splitting due to the perturbationintroduced by the additional oxygen layer.

FIG. 5B shows the calculated band structure from the Z point for bothbulk silicon (continuous lines) and for the 4/1 Si/O superlattice 25(dotted lines). This figure illustrates the enhanced curvature of thevalence band in the (100) direction.

FIG. 5C shows the calculated band structure from the both the gamma andZ point for both bulk silicon (continuous lines) and for the 5/1/3/1Si/O structure of the superlattice 25′ of FIG. 4 (dotted lines). Due tothe symmetry of the 5/1/3/1 Si/O structure, the calculated bandstructures in the (100) and (010) directions are equivalent. Thus theconductivity effective mass and mobility are expected to be isotropic inthe plane parallel to the layers, i.e. perpendicular to the (001)stacking direction. Note that in the 5/1/3/1 Si/O example the conductionband minimum and the valence band maximum are both at or close to the Zpoint.

Although increased curvature is an indication of reduced effective mass,the appropriate comparison and discrimination may be made via theconductivity reciprocal effective mass tensor calculation. This leadsApplicants to further theorize that the 5/1/3/1 superlattice 25′ shouldbe substantially direct bandgap. As will be understood by those skilledin the art, the appropriate matrix element for optical transition isanother indicator of the distinction between direct and indirect bandgapbehavior.

Referring now additionally to FIGS. 6A-6E, a method for making theMOSFET 20 will now be described. The method begins with providing thesilicon substrate 21. By way of example, the substrate may be aneight-inch wafer 21 of lightly doped P-type or N-type single crystalsilicon with <100> orientation, although other suitable substrates mayalso be used. In accordance with the present example, a layer of thesuperlattice 25 material is then formed across the upper surface of thesubstrate 21.

More particularly, the superlattice 25 material is deposited across thesurface of the substrate 21 using atomic layer deposition and theepitaxial silicon cap layer 52 is formed, as discussed previously above,and the surface is planarized to arrive at the structure of FIG. 6A. Itshould be noted that in some embodiments the superlattice 25 materialmay be selectively deposited in those regions where channels are to beformed, rather than across the entire substrate 21, as will beappreciated by those skilled in the art. Moreover, planarization may notbe required in all embodiments.

The epitaxial silicon cap layer 52 may have a preferred thickness toprevent superlattice consumption during gate oxide growth, or any othersubsequent oxidations, while at the same time reducing or minimizing thethickness of the silicon cap layer to reduce any parallel path ofconduction with the superlattice. According to the well knownrelationship of consuming approximately 45% of the underlying siliconfor a given oxide grown, the silicon cap layer may be greater than 45%of the grown gate oxide thickness plus a small incremental amount toaccount for manufacturing tolerances known to those skilled in the art.For the present example, and assuming growth of a 25 angstrom gate, onemay use approximately 13-15 angstroms of silicon cap thickness.

FIG. 6B depicts the MOSFET 20 after the gate oxide 37 and the gateelectrode 36 are formed. More particularly, a thin gate oxide isdeposited, and steps of poly deposition, patterning, and etching areperformed. Poly deposition refers to low pressure chemical vapordeposition (LPCVD) of silicon onto an oxide (hence it forms apolycrystalline material). The step includes doping with P+ or As− tomake it conducting, and the layer may be around 250 nm thick, forexample.

In addition, the pattern step may include performing a spinningphotoresist, baking, exposure to light (i.e., a photolithography step),and developing the resist. Usually, the pattern is then transferred toanother layer (oxide or nitride) which acts as an etch mask during theetch step. The etch step typically is a plasma etch (anisotropic, dryetch) that is material selective (e.g., etches silicon ten times fasterthan oxide) and transfers the lithography pattern into the material ofinterest.

Referring to FIG. 6C, once the gate 35 is formed, the gate may then beused as an etch mask to remove the superlattice 25 material and portionsof the substrate 21 in the regions where the source and drain are to beformed, as will be appreciated by those skilled in the art. As may beseen, this step also forms the underlying portion 24 of the substrate21. The superlattice 25 material may be etched in a similar fashion tothat described above for the gate 35. However, it should be noted thatwith the non-semiconductor present in the superlattice 25, e.g., oxygen,the superlattice may be more easily etched using an etchant formulatedfor oxides rather than silicon. Of course, the appropriate etch for agiven implementation will vary based upon the structure and materialsused for the superlattice 25 and substrate 21, as will be appreciated bythose of skill in the art.

In FIG. 6D, lightly doped source and drain (“LDD”) extensions 22, 23 areformed. These regions are formed using n-type or p-type LDDimplantation, annealing, and cleaning. An anneal step may be used afterthe LDD implantation, but depending on the specific process, it may beomitted. The clean step is a chemical etch to remove metals and organicsprior to depositing an oxide layer.

FIG. 6E shows the formation of the sidewall spacers 40, 41 and thesource and drain 26, 27 implants. An SiO₂ mask is deposited and etchedback. N-type or p-type ion implantation is used to form the source anddrain regions 26, 27. The structure is then annealed and cleaned.Self-aligned silicide formation may then be performed to form thesilicide layers 30, 31, and 34, and the source/drain contacts 32, 33,are formed to provide the final semiconductor device 20 illustrated inFIG. 1. The silicide formation is also known as salicidation. Thesalicidation process includes metal deposition (e.g. Ti), nitrogenannealing, metal etching, and a second annealing.

The foregoing is, of course, but one example of a process and device inwhich the present invention may be used, and those of skill in the artwill understand its application and use in many other processes anddevices. In other processes and devices the structures of the presentinvention may be formed on a portion of a wafer or across substantiallyall of a wafer. Additionally, the use of an atomic layer deposition toolmay also not be needed for forming the superlattice 25 in someembodiments. For example, the monolayers may be formed using a CVD toolwith process conditions compatible with control of monolayers, as willbe appreciated by those skilled in the art.

An alternate embodiment of the semiconductor device 20″ and method formmaking the same will now be described with reference to FIGS. 7A-7D and8. The steps performed in FIGS. 7A-7B are the same as those discussedabove with reference to FIGS. 6A-6B, except that the spaces 40″ areformed before patterning the superlattice 25″ and substrate 21″. Theyare instead patterned to extend laterally beyond the edges of the gatestack.

The lightly doped source/drain extension regions 22″, 23″ and thesource/drain regions 26″, 27″ are then formed, as previously discussedabove and shown in FIGS. 7C-7D. Yet, the resulting structure will differfrom the semiconductor device 20 in that the superlattice 25 channel isaligned with the sidewall spacers 40″, 41″ rather than the edges of thegate 35″ stack. This embodiment may be preferred in some applications,as current flow between the lightly doped source/drain regions 22, 23 ofthe semiconductor device 20 will make a vertical transition thatapproaches ninety degrees, whereas in the present embodiment thevertical transition will occur more gradually. Thus, these two differentconfigurations may be used to provide different device characteristicsfor different applications, as will be appreciated by those skilled inthe art.

A completed semiconductor device 20″ formed using the method illustratedin FIGS. 7A-7D is shown in FIG. 8. Here, multiple MOSFETS are formed inthe substrate 21″, such as NMOS and PMOS transistors to provide a CMOSdevice. More particularly, shallow trench isolation (STI) regions 80″may be formed between adjacent MOSFETS, as will be appreciated by thoseskilled in the art. In accordance with one embodiment, the STI regions80″ may be formed prior to depositing the superlattice 25″, so that theSTI regions thus provide boundaries for selective deposition of thesuperlattice.

More particularly, the wafer is patterned and trenches are etched (e.g.,0.3-0.8 um) in the desired STI regions. A thin oxide is then grown, andthe trenches are filled with SiO₂ to provide the STI regions 80″, andthe upper surfaces thereof may be planarized, if desired. The STIregions 80″ may also be used as an etch stop in performing certain ofthe above-noted steps, as will be appreciated by those skilled in theart. The superlattice 25″ structure may also be formed prior toformation of the STI regions 80″ to thereby eliminate a masking step, ifdesired. Further details regarding fabrication of the semiconductordevices in accordance with the present invention may be found in theabove-noted U.S. application Ser. No. 10/467,069.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A semiconductor device comprising: a semiconductor substrate; and atleast one metal oxide semiconductor field-effect transistor (MOSFET)comprising spaced apart source and drain regions in said semiconductorsubstrate, a superlattice channel comprising a plurality of stackedgroups of layers on said semiconductor substrate between said source anddrain regions, and having upper surface portions being verticallystepped above adjacent upper surface portions of said source and drainregions, each group of layers of said superlattice channel comprising aplurality of stacked base semiconductor monolayers defining a basesemiconductor portion and an energy band-modifying layer thereon, saidenergy-band modifying layer comprising at least one non-semiconductormonolayer constrained within a crystal lattice of adjacent basesemiconductor, and a gate overlying said superlattice channel.
 2. Thesemiconductor device of claim 1 wherein said semiconductor substratecomprises an underlying portion aligned with said superlattice channel.3. The semiconductor device of claim 1 wherein said gate comprises agate oxide layer and a gate electrode thereover; and wherein saidsuperlattice channel is aligned with said gate electrode.
 4. Thesemiconductor device of claim 1 wherein said gate comprises a gate oxidelayer, a gate electrode thereover, and sidewall spacers on opposingsides of said gate electrode; and wherein said superlattice channel isaligned with said sidewall spacers.
 5. The semiconductor device of claim1 wherein said at least one MOSFET comprises a plurality thereof; andfurther comprising an isolation region in said semiconductor substratebetween adjacent MOSFETs.
 6. The semiconductor device of claim 1 furthercomprising a contact layer on at least one of said source and drainregions.
 7. The semiconductor device of claim 1 wherein saidsuperlattice channel has a common energy band structure therein.
 8. Thesemiconductor device of claim 1 wherein said superlattice channel has ahigher charge carrier mobility than would otherwise be present withoutsaid non-semiconductor layer.
 9. The semiconductor device of claim 1wherein each base semiconductor portion comprises silicon.
 10. Thesemiconductor device of claim 1 wherein each base semiconductor portioncomprises germanium.
 11. The semiconductor device of claim 1 whereineach energy band-modifying layer comprises oxygen.
 12. The semiconductordevice of claim 1 wherein each energy band-modifying layer is a singlemonolayer thick.
 13. The semiconductor device of claim 1 wherein eachbase semiconductor portion is less than eight monolayers thick.
 14. Thesemiconductor device of claim 1 wherein said superlattice further has asubstantially direct energy bandgap.
 15. The semiconductor device ofclaim 1 wherein said superlattice further comprises a base semiconductorcap layer on an uppermost group of layers.
 16. The semiconductor deviceof claim 1 wherein all of said base semiconductor portions are a samenumber of monolayers thick.
 17. The semiconductor device of claim 1wherein at least some of said base semiconductor portions are adifferent number of monolayers thick.
 18. The semiconductor device ofclaim 1 wherein each energy band-modifying layer comprises anon-semiconductor selected from the group consisting of oxygen,nitrogen, fluorine, and carbon-oxygen.
 19. A semiconductor devicecomprising: a semiconductor substrate; and at least one metal oxidesemiconductor field-effect transistor (MOSFET) comprising spaced apartsource and drain regions in said semiconductor substrate, a superlatticechannel comprising a plurality of stacked groups of layers on saidsemiconductor substrate between said source and drain regions, andhaving upper surface portions being vertically stepped above adjacentupper surface portions of said source and drain regions, each group oflayers of said superlattice channel comprising a plurality of stackedbase semiconductor monolayers defining a base semiconductor portion andan energy band-modifying layer thereon, said energy-band modifying layercomprising at least one non-semiconductor monolayer constrained within acrystal lattice of adjacent base semiconductor, and a gate comprising agate oxide layer overlying said superlattice channel and a gateelectrode thereover, and said gate electrode being aligned with saidsuperlattice channel; said semiconductor substrate comprising anunderlying portion aligned with said superlattice channel.
 20. Thesemiconductor device of claim 19 wherein said at least one MOSFETcomprises a plurality thereof; and further comprising an isolationregion in said semiconductor substrate between adjacent MOSFETs.
 21. Thesemiconductor device of claim 19 further comprising a contact layer onat least one of said source and drain regions.
 22. The semiconductordevice of claim 19 wherein said superlattice channel has a common energyband structure therein.
 23. The semiconductor device of claim 19 whereinsaid superlattice channel has a higher charge carrier mobility thanwould otherwise be present without said non-semiconductor layer.
 24. Thesemiconductor device of claim 19 wherein each base semiconductor portioncomprises silicon.
 25. The semiconductor device of claim 19 wherein eachbase semiconductor portion comprises germanium.
 26. The semiconductordevice of claim 19 wherein each energy band-modifying layer comprisesoxygen.
 27. The semiconductor device of claim 19 wherein each energyband-modifying layer is a single monolayer thick.
 28. The semiconductordevice of claim 19 wherein each base semiconductor portion is less thaneight monolayers thick.
 29. The semiconductor device of claim 19 whereinsaid superlattice further has a substantially direct energy bandgap. 30.The semiconductor device of claim 19 wherein said superlattice furthercomprises a base semiconductor cap layer on an uppermost group oflayers.
 31. The semiconductor device of claim 19 wherein all of saidbase semiconductor portions are a same number of monolayers thick. 32.The semiconductor device of claim 19 wherein at least some of said basesemiconductor portions are a different number of monolayers thick. 33.The semiconductor device of claim 19 wherein each energy band-modifyinglayer comprises a non-semiconductor selected from the group consistingof oxygen, nitrogen, fluorine, and carbon-oxygen.
 34. A semiconductordevice comprising: a semiconductor substrate; and at least one metaloxide semiconductor field-effect transistor (MOSFET) comprising spacedapart source and drain regions in said semiconductor substrate, asuperlattice channel comprising a plurality of stacked groups of layerson said semiconductor substrate between said source and drain regions,and having upper surface portions being vertically stepped aboveadjacent upper surface portions of said source and drain regions, eachgroup of layers of said superlattice channel comprising a plurality ofstacked base semiconductor monolayers defining a base semiconductorportion and an energy band-modifying layer thereon, said energy-bandmodifying layer comprising at least one non-semiconductor monolayerconstrained within a crystal lattice of adjacent base semiconductor, anda gate comprising a gate oxide layer overlying said superlatticechannel, a gate electrode thereover, and sidewall spacers on opposingsides of said gate electrode, said sidewall spacers being aligned withsaid superlattice channel; said semiconductor substrate comprising anunderlying portion aligned with said superlattice channel.
 35. Thesemiconductor device of claim 34 wherein said at least one MOSFETcomprises a plurality thereof; and further comprising an isolationregion in said semiconductor substrate between adjacent MOSFETs.
 36. Thesemiconductor device of claim 34 further comprising a contact layer onat least one of said source and drain regions.
 37. The semiconductordevice of claim 34 wherein said superlattice channel has a common energyband structure therein.
 38. The semiconductor device of claim 34 whereinsaid superlattice channel has a higher charge carrier mobility thanwould otherwise be present without said non-semiconductor layer.
 39. Thesemiconductor device of claim 34 wherein each base semiconductor portioncomprises silicon.
 40. The semiconductor device of claim 34 wherein eachbase semiconductor portion comprises germanium.
 41. The semiconductordevice of claim 34 wherein each energy band-modifying layer comprisesoxygen.
 42. The semiconductor device of claim 34 wherein each energyband-modifying layer is a single monolayer thick.
 43. The semiconductordevice of claim 34 wherein each base semiconductor portion is less thaneight monolayers thick.
 44. The semiconductor device of claim 34 whereinsaid superlattice further has a substantially direct energy bandgap. 45.The semiconductor device of claim 34 wherein said superlattice furthercomprises a base semiconductor cap layer on an uppermost group oflayers.
 46. The semiconductor device of claim 34 wherein all of saidbase semiconductor portions are a same number of monolayers thick. 47.The semiconductor device of claim 34 wherein at least some of said basesemiconductor portions are a different number of monolayers thick. 48.The semiconductor device of claim 34 wherein each energy band-modifyinglayer comprises a non-semiconductor selected from the group consistingof oxygen, nitrogen, fluorine, and carbon-oxygen.